43 6 On the Source of the Rontgen Rays in Focus Tubes. 



fluorescent portions of the glass of the tube can be distinctly seen. 

 Further, it is noticeable that that portion of the glass that shows the 

 brightest fluorescence, i.e., that part which lies in the path in which 

 cathode rays would be reflected from the anti-cathode surface were 

 they reflected according to the law of equal angles of incidence and 

 reflection gives off the most Rontgen rays, while those portions of 

 the glass that show no fluoresence do not give off any Rontgen. rays. 

 The conclusion appears obvious that whatever produces the one also 

 produces the other, but as has been pointed out by Professor S. P. 

 Thompson* and others, the fluorescence is not due to the direct stream 

 of rays from the cathode, which cannot reach portions of the glass that 

 show fluorescence, but to some description of radiation that proceeds 

 from the surface of the anti-cathode that faces the cathode. In the 

 paper above referred to Professor Thompson calls these radiations 

 " para-cathodic rays," stating that they differ from the Rontgen rays 

 in respect of their power of penetration, and in their capacity of 

 being electrostatically and magnetically deflectable. In these re- 

 spects the writer's experiments confirm those of Professor Thompson, 

 but when the latter goes on to differentiate these rays from ordi- 

 nary cathode rays, on account of their not exciting Rontgen rays 

 where they impinge on a solid surface, the writer is unable to 

 agree, for, as above stated, these rays do excite Rontgen rays where 

 they impinge upon the glass walls of the tube ; as mentioned, however, 

 they do this only to an extent that is relatively very feeble, and so 

 far as the author knows only discernable by the pin-hole method of 

 observation, which no doubt explains Professor Thompson's failure 

 to observe the effect. The " para-cathodic " radiations in question do 

 not, however, appear to be ordinary cathode rays. In the first place 

 they do not proceed directly from the cathode, but only from the 

 surface of the anti- cathode that faces the latter. Secondly, they do 

 not appear to be negatively but positively charged, as can be ascer- 

 tained by means of an exploring pole connected with an electro- 

 scope. The writer suggests that, assuming the correctness of the 

 Crookes theory of the nature of the cathode rays, these "para- 

 cathodic " rays may very probably consist of cathode ray particles 

 which, having struck the anti-cathode, and having thus given up 

 their negative charges and acquired positive charges, rebound, both 

 by reason of their elasticity and also by repulsion from the anti- 

 cathode. Perhaps owing to the comparative roughness of the anti- 

 cathode surface, they fly off to some extent in all available directions, 

 but they do so especially in that direction which the law of equal angles 

 of incidence and reflection requires. It also appears very possible 

 that these " paracathodic " rays are identical with the positively 

 electrified streams proceeding from the anode, which the writer has 

 * See 'Phil. Trans.,' A., vol. 190, pp. 471490. 



